1. Field of the Invention
The invention pertains to the field of electrical equipment manufacture and, particularly, the electrical attachment of integrated circuit chips, such as microprocessors and application-specific integrated circuits (ASICS), to printed circuit boards. More specifically, the method of attachment uses a bolster plate to reinforce the printed circuit board during the attachment process.
2. Discussion of the Related Art
A variety of methods have been devised for the attachment of integrated circuit chips, such as microprocessors and ASCIS, to printed circuit boards in a manner that assures consistency in establishing good electrical contact. Many of these methods involve the use of high compressive loads, which are applied to the processors. The integrated circuit chips typically have a plurality of pins that mate with a corresponding female conductive receptacle in the printed circuit board. The printed circuit boards, alone, lack sufficient rigidity to support the compressive loads during attachment. For example, these loads may range from two hundred to three hundred pounds force (890 to 1300 Newtons). Resultant bending of the printed circuit board is capable of damaging wiring or other materials within the integrated circuit chip. Additionally, the bending moment is capable of disrupting the desired electrical contact.
The problem of printed circuit board bending under these heavy loads is typically resolved by using a bolster plate, which is a piece of metal that attaches to the printed circuit board, e.g., by bolting, riveting, or adhesion. The bolster plate may be constructed in any geometrical shape that provides the requisite support. The bolster plate is usually located on the reverse side of the printed circuit board opposite that side on which the integrated circuit chip resides.
Newer microprocessors and ASICS devices have increased numbers of pins in comparison to older devices. Furthermore, the newer devices operate at much higher speeds than did older devices. The increasing number of pins and higher levels of performance demand closer mechanical tolerances for manufacturing purposes. It has been discovered that the use of a bolster plate according to traditional practices does not sufficiently eliminate the bending moment in the printed circuit boards in light of these new demands. In applications where, for example, a force of 270 pounds
(1200 Newtons) is applied to
seat a microprocessor, a conventional bolster plate may bow a distance of 0.001 inch (0.0025 cm). Even this small amount of bending is sufficient damage the assembly or to cause failure in the electrical contact.
The bolster bow or bend is at maximum in the center of the bolster plate. Additional rigidity could be imparted by increasing the thickness of the bolster plate, but this requires additional room for the bolster plate. The increased thickness creates other difficulties in the context of fitting additional components on the printed circuit board and in assembling adjacent components in the intended use environment. Attempts have been made to pre-bow or pre-stress the bolster plate to accommodate the stress during the attachment of microprocessors, but the resulting bending moment from pre-stressing the bolster plate was not repeatable.
There remains a problem in preventing bolster plate bending due to the insertion of newer microprocessors and ASICS devices.
A bolster plate according to the principles described herein overcomes the problems described above and advances the art by providing a method, apparatus and software pertaining to a shim or shim assembly that compensates the bolster plate for bending deformations during the attachment of integrated circuit chips. The shim may be located, for example, at a position where the maximum amount of deformation occurs under load from the attachment process. Thus, the shim substantially fills the deformation under load and prevents damage to the microprocessor by providing support to the assembly preventing corresponding deformation in the integrated circuit chip, notably, in the pins, wiring and silicon, which are subject to breakage under small amounts of deformation.
A bolster plate according to these principles is used for supporting a printed circuit board during attachment of an integrated circuit chip to the printed circuit board. The bolster plate comprises a support rail presenting a contact face for use in contacting the printed circuit board. The rail demarcates a central well that contains a platform presenting a support surface configured to support a selected portion of the printed circuit board underneath the integrated circuit chip during attachment of the integrated circuit chip to the printed circuit board. Where the bolster plate is made out of a metal, an insulator preferably covers the support surface. A shim is interposed between the insulator and the support surface where the insulator is required to prevent short circuiting of the integrated circuit chip. With or without the insulator, the shim is positioned at a point or points of maximum deformation in the bolster plate. The dimensions of the shim are preselected to compensate for deformation of the bolster plate under the design load by filling the point or points of maximum deformation.
While the dimensions of the shim may be determined by trial and error, a much preferred manner of determining the shim dimensions is to calculate, e.g., through finite element mathematical modeling, the predetermined dimensions that are operable to compensate for bending of the bolster plate under a maximum applied load during attachment of the integrated circuit chip. This modeling assures that the deformed support surface under load is shim-compensated to present a total deformation of less than, for example, a 0.001 inch or 0.0005 inch (0.0025 or 0.0038 cm) bow at a center of the bolster plate under the maximum applied load. The term xe2x80x9cfinite element modelingxe2x80x9d is hereby defined to include both finite analysis and finite difference modeling techniques.
The shim may have any geometrical shape, such as a square or rectangular shape, but a disk or ovaloid shape is preferred for correspondence with the shape of bow deformation in the bolster plate. Particularly preferred shims comprise a plurality of pieces, such as two disks, where the pieces have different dimensions and are concentrically stacked to present a stair-stepped edge providing a transition to the support surface that is less abrupt than a non-tapered shim. Alternatively, a single shim may be tapered or machined to have a stair-step, in order to ease the transition.
The bolster plate that is described above may be used in a method of attaching an integrated circuit chip to a chip-mounting receptacle in a printed circuit board. The method comprises the steps of assembling a bolster plate including the shim, attaching the bolster plate to the printed circuit board; and pressing the integrated circuit chip into the chip mounting receptacle. Further method steps preferably but optionally comprises modeling a bending moment in the bolster plate under a maximum applied load for use in the step of pressing the integrated circuit chip to provide model results for shim-based compensation of the bending moment, and selecting dimensions of the shim based upon the model results.
The principles described herein also pertain to a computer readable form comprising machine instructions that are operable for determining a bow deformation in the bolster plate when the bolster plate is placed under a maximum load during attachment of an integrated circuit chip, and identifying dimensions for the shim that may be used to compensate for the bow deformation. In a manufacturing environment, permits the selective adjustment of shim dimensions to compensate for bow deformation on the basis of different bolster plate designs and materials, as well as different applied loads. If an increase in device failure rate is traceable to the chip attachment process, the shim dimensions can be selectively adjusted to overcome the observed failure increases.